Trojan horses can be considered as a kind of spyware. The Trojan horse infection method to the computer is very similar to the virus infection method.
Opening a link that comes by an e-mail or downloading a file from internet that we are not sure of its security of the source and opening it result in with infecting our computer with a Trojan. But contrary to viruses, Trojan horses may not have visible damages on the system. Te most obvious thing that Trojan horses do on a computer is to open a TCP or UDP port, which
allows another malicious entity to reach the entire system through these ports.